The present invention relates to an electromagnetic actuator for moving an object, e.g., an objective lens of an optical head, on a micro level, and more particularly to an electromagnetic actuator having specific coil arrangement for improving the utilization rate of magnetic circuit so as to quickly perform the micro-level movement of the object.
In general, the writing and reading operations of an information writing/reading apparatus are performed by a disk-shaped recording medium such as a compact disk or a magneto-optical disk. For reading information from a disk, an optical head including an objective lens is used to guide the laser beam reflected off of the disk to a photosensor.
Unfortunately, a general disk is hard to be perfectly flat and symmetric, i.e. it may have inevitable warpage and eccentricity to some extent. Therefore, the information writing/reading apparatus preferably includes means for moving the objective lens in response to the deviations resulting from the variable warpage and/or eccentricity of the disk. The movement is generally performed on a micro level along two orthogonal directions, i.e. a focusing direction perpendicular to the disk face and a tracking direction parallel to the disk face, to compensate the focusing deviation and the tracking deviation, respectively.
In order to move the objective lens on a micro level, objective lens actuators are developed. For the actuators using electromagnetic forces as driving power, a tracking coil, a focusing coil and a permanent magnet are involved and specifically arranged.
Please refer to FIGS. 1, 2 and 3 which schematically show three kinds of conventional arrangement of the tracking coil 11, the focusing coil 12 and the permanent magnet 13. According to the proportion of the magnetic force lines passing through the coils, the utilization rate of the magnetic circuit of FIG. 1 or 2 is estimated to be 30% to 40%, and that of FIG. 3 is about 50%.
With the enhancement of writing and reading speeds, the tracking and focusing operations are required to be highly responsive. If the utilization rate of the magnetic circuit is relatively low, the driving power is not enough for the application of high-speed writing and reading operations.
Therefore, an object of the present invention is to provide an electromagnetic actuator with improved responsive speed by increasing the utilization rate of the magnetic circuit, which has a particular coil arrangement.
According to a first aspect of the present invention, an electromagnetic actuator for moving an object along a first direction includes a first actuating coil set connected to the object for generating a first actuating force to move the object in the first direction in response to a first current density therein and a first magnetic force; and a magnetic force line generator surrounded by the first actuating coil set, and including two homopolar parts disposed with a clearance small enough for generating magnetic force lines including a first substantially linear portion due to a repelling force between the two homopolar parts, wherein the first substantially linear portion of magnetic force lines provides the first magnetic force for the first actuating coil set.
Preferably, the magnetic force line generator includes two permanent magnets having respective homopoles facing each other as the two homopolar parts.
In an embodiment, the first direction is a focusing direction, and the first actuating coil set includes a coil holding the magnetic force line generator therein.
Preferably, the electromagnetic actuator further moves the object along a tracking direction perpendicular to the focusing direction. Therefore, the electromagnetic actuator preferably further includes a second actuating coil set connected to the object for generating a second actuating force to move the object in the tracking direction in response to a second current density therein and a second magnetic force. The magnetic force lines further includes a second substantially linear portion due to the repelling force between the two homopolar parts, wherein the second substantially linear portion of magnetic force lines provides the second magnetic force for the second actuating coil set.
Preferably, the second actuating coil set consists of two coils disposed at two opposite sides of the magnetic force line generator, respectively, each of which has a coil wall thereof substantially perpendicular to the second substantially linear portion.
In another embodiment, the first direction is a tracking direction, and the first actuating coil set includes two coils sandwiching the magnetic force line generator therebetween.
Preferably, the electromagnetic actuator further moves the object along a focusing direction perpendicular to the tracking direction. Therefore, the electromagnetic actuator preferably further includes a second actuating coil set connected to the object for generating a second actuating force to move the object in the focusing direction in response to a second current density therein and a second magnetic force. The magnetic force lines further includes a second substantially linear portion due to the repelling force between the two homopolar parts, wherein the second substantially linear portion of magnetic force lines provides the second magnetic force for the second actuating coil set.
Preferably, the second actuating coil set includes a coil holding the magnetic force line generator therein, which has a coil wall thereof substantially perpendicular to the second substantially linear portion.
According to a second aspect of the present invention, an electromagnetic actuator is provided for moving an object along a first direction and a second direction. The electromagnetic actuator includes a magnetic force line generator including two homopolar parts spaced with a clearance small enough for generating magnetic force lines including a first substantially linear portion and a second substantially linear portion due to a repelling force between the two homopolar parts, wherein the first and second substantially linear portions are not parallel; a first actuating coil set connected to the object and arranged around the magnetic force line generator with a coil wall thereof substantially perpendicular to the first substantially linear portion for moving the object in the first direction in response to a first current density therein and the first substantially linear portion; and a second actuating coil set connected to the object and located at the lateral end of the magnetic force line generator with a coil wall thereof substantially perpendicular to the second substantially linear portion for moving the object in the second direction in response to a second current density and the second substantially linear portion, wherein the first and second actuating coil sets are not substantially parallel.
According to a third aspect of the present invention, an electromagnetic actuator for moving an object along a first direction and a second direction includes a first actuating coil set connected to the object for generating a first actuating force to move the object in the first direction in response to a first current therein and a first magnetic force; a second actuating coil set connected to the object for generating a second actuating force to move the object in the second direction in response to a second current therein and a second magnetic force, wherein the first and second actuating coil sets are not substantially parallel; and a magnetic force line generator including two spaced homopolar parts which is surrounded by the first actuating coil set, and sandwiched by the second actuating coil set for providing the first and second magnetic forces for the first and second actuating coil sets, respectively, by generating magnetic force lines due to a repelling force therebetween, wherein the first and second magnetic forces are not substantially parallel.